Such axle alignment systems have the distinction of a contactless optical aligning of targets mounted on the vehicle wheels. The targets have optically recordable marks which are recorded by picture-taking devices, and the images of the marks are evaluated by an evaluation unit using the means of image processing.
A method and a device having a stereo camera device as the image-taking devices is discussed in DE 42 16 426 C1, in which, outside the axle of a wheel of the motor vehicle, at the end face of the wheel, an optically recordable mark is applied. For the determination of the spatial position of the wheel rotational axis, the wheels are put in rotation, and during the rotational motion, the mark rotating about its axle is recorded by the stereo cameras of the image-taking device in successive images. The spatial position of the axle of the wheel is ascertained by the evaluation unit from the recorded pictures of the mark. During the measuring process, this axle alignment system does not draw upon any local 3D coordinate system for the marks, so that, in order to carry out each new measurement, the wheel that is to be measured, has to be put in rotation anew.
Another axle alignment system having a stereo camera device is discussed in DE 197 57 763 A1. Besides the marks for the wheel of the motor vehicle, a separate reference feature device having reference features is used. Using a separate calibration process on rotating wheels, a local 3D coordinate system and the wheel's rotational axis are ascertained, using the marks on the wheel, for the compensation for the rim runout. For a wheel standing still, the measuring process takes place by recording the marks on the wheel and the reference features of the reference feature device. The recording is evaluated using the evaluation unit, and using known methods of image processing and triangulation.
In these axle alignment systems using stereo camera devices, the measuring units having stereo cameras are in each case situated laterally next to the wheels, so that the field of vision of the cameras is directed, transversely to the vehicle longitudinal center plane, at the optically recordable marks present there or the adaptor affixed to the end face of the wheel as mark carrier, the so-called targets.
In an axle alignment system of WO 94/05969 A1, a mono-camera device is present whose field of vision is directed, in the direction of the vehicle longitudinal center plane, at targets mounted on the wheels, the targets being laterally distant in the space from wheels and extending into the field of vision of the cameras. On the targets, a plurality of marks have been applied which have known geometrical features and relationships that are known to the evaluation unit. During the measuring process, the marks are recorded by the image-recording device, using a single image. By a comparison of the geometry and relationships of the recorded marks to the geometrical features and relationships of the marks known to the evaluation unit, the evaluation unit determines the orientation of the target, and from this the spatial position of the rotational axis of the wheel is ascertained.